Composite materials have shown commercial utility as replacements for structural materials used to make structural components, especially as replacements for metals that typically have a greater material density and therefore weigh more than do lighter composite materials.
Composite materials are typically made by combining or “laying up” sequential layers, or laminates, of a fiber-containing material that is impregnated with a resin-material, followed by curing. Such impregnated fiber layers, or “prepregs” typically comprise a resin that serves, among other functions, to adhere the prepreg layers together upon curing, with such curing typically accomplished at temperatures and pressures that are greater than ambient temperatures and pressures.
In the field of aeronautics, airplanes and other spacecraft typically comprise structural components made from adhesively joined composite materials, although adhesively joined composite materials also can be used to make other vehicles and objects. Adhesive bonding layers between adhered composites, or between a composite and other material parts, especially post-cure, cannot be observed or inspected without destroying the bond, as well as the components made by parts by adhesively joined parts. Therefore, determining the strength of an adhesive bond in a component comprising adhesive bonds has required mechanically testing the bond invasively, resulting in the destruction of the part as mechanical stress-based bond testing is conducted.
In the aeronautical field, regulatory concern over the incomplete bonding of adhesively joined composite materials has led to regulations requiring the use and placement of mechanical fasteners at the location of adhesively joined composite parts. Installing such fasteners to satisfy regulatory mandates increases overall labor, production time and cost for the manufacturing methods of structures comprising joined composite materials, as well as increasing weight and cost of the resulting manufactured structure comprising the adhesively joined composite materials.
Practical, non-destructive inspection methods of adhesively joined composite materials have not been devised, or have not yet been accepted by regulatory authorities. Further, inspection methods of adhesively joined composite materials that are destructive (e.g., the adhesively joined composite parts must be destroyed to conduct inspection and determine satisfactory bonding of the adhesively joined composite parts) render the adhesively joined composites, or parts made from the adhesively joined composites, useless after inspection, and would therefore add to composite material waste. Such composite material waste leads to increased cost, and also leads to increases in production time, and/or production delays that can further result in cost increases.